1. Field of the Invention
The present invention relates to x-ray diagnostic installations, and in particular to x-ray diagnostic installations having a video display chain.
2. Description of the Prior Art
X-ray diagnostic installations wherein the display means, such as a video display system or chain, are known which employ a means for generating subtraction images. Such subtraction images are particularly useful in angiography for highlighting blood vessels which are otherwise difficult to perceive in the normal x-ray picture, or which may be covered by bone structure.
An x-ray diagnostic installation for producing subtraction images is described in U.S. Pat. 4,398,213 wherein a blank image, that is, an image without the presence of a contrast agent, is obtained and averaged over a plurality of scannings, the blank image being stored in a memory. Subsequently the person being examined is injected with a contrast agent. Images generated with contrast agent present are then either directly supplied to a difference forming stage (transillumination subtraction) or are deposited in a further image memory averaged over the same plurality of scannings. Subtraction of the blank image from the contrast agent image is undertaken in the difference forming stage, so that only the vessels filled with contrast agent which are of interest can be subsequently seen on a monitor.
Averaging of the video picture is undertaken for the purpose of reducing the picture noise. A circuit for suppressing picture noise is described in detail in U.S. Pat. No. 4,064,530. In this known system, noise suppression is achieved by weighting the current video signal with a factor (1-a) and weighting a stored video signal with the factor (a). The two weighted video signals are added and are entered into the image memory. Given constant factors, (1-a) and (a), the noise component will be too high if the value of (a) is selected too small, whereas rapid motion sequences cannot be perceived if the value of (a) is selected too large. The system disclosed in U.S. Pat. No. 4,064,530 undertakes a difference formation of successive video signal components in order to recognize movement in the video signal and to correspondingly select the value of (a) low given occurring large motion, and to correspondingly increase the value of (a) given slight motion.
If such a conventional circuit were employed in angiography, the large modification of the video signal which occurs upon the appearance of the contrast agent would erroneously be interpreted by this known circuit as motion, and thus would result in the selection of a low factor (a). Although the chronological progression of the contrast agent could be displayed without delay, the image would be extremely noisy particularly in those portions thereof outside of the blood vessels.